Optimization of solid-state fermentation for production of cellulase from agricultural residues

Introduction

The enzymatic breakdown of cellulose into fermentable sugars sits at the heart of secondgeneration bioethanol production, yet the high cost of commercial cellulase preparations remains a barrier to economic viability [1] . Solid-state fermentation (SSF) offers a costeffective alternative to submerged fermentation for cellulase production because it uses cheap lignocellulosic residues as both the carbon source and the physical support for fungal growth, and it operates at lower water volumes, reducing downstream processing costs [2] . Nigeria generates over 160 million tonnes of crop residues annually; cassava peels and rice straw together account for roughly 60 million tonnes, much of which is burned in the field [3] . Wheat bran and sugarcane bagasse are likewise produced in surplus by milling industries. Filamentous fungi-particularly Aspergillus niger and Trichoderma reesei-are the most widely used cellulase producers under SSF because they secrete a complete enzyme system (endoglucanase, exoglucanase, β-glucosidase) and tolerate low water activity [4] . However, enzyme yields are sensitive to fermentation variables such as pH, temperature, moisture content, and nitrogen supplementation, and optimum conditions differ with both the organism and the substrate [5] . Earlier reports optimised SSF for cellulase using individual substrates, but comparative data across multiple residues under identical conditions are less common, especially for the humid tropical climate of southwestern Nigeria [6] . This research therefore compared rice straw, wheat bran, and sugarcane bagasse as substrates for cellulase production by A. niger MTCC 2196 under SSF and optimised key process parameters to maximise enzyme yield.

Rice straw was collected from the university farm, wheat bran from a local flour mill in Abeokuta, and sugarcane bagasse from a nearby sugar factory in Ogun State, all in Abeokuta district. Substrates were oven-dried at 60 °C, ground to pass a 1 mm sieve, and autoclaved (121 °C, 20 min) before use. Chemicals (carboxymethylcellulose, DNS reagent, yeast extract) were of analytical grade (Oxoid, UK) [7] .

Methods

SSF was carried out in 250 mL Erlenmeyer flasks containing 10 g of each substrate moistened with Mandels' mineral salt solution to the desired moisture level and inoculated with 1 mL spore suspension (1×10⁷ spores mL⁻¹).

Flasks were incubated at 30 °C (baseline) for 12-96 h. Enzyme extraction used 50 mL of 0.05 M citrate buffer (pH 4.8) with agitation at 150 rpm for 30 min, followed by filtration and centrifugation (10,000 × g, 15 min). Cellulase (filter paper activity, FPase) was assayed by the method of Ghose [8] and expressed as IU g⁻¹ dry substrate. Optimisation followed a one-factor-at-a-time approach: pH (4.0-6.5), temperature (25-40 °C), moisture (45-80%), and nitrogen supplement (yeast extract 0.5-2.0%). All experiments were run in triplicate. Data were analysed by one-way ANOVA and Tukey's test (p<0.05) using R v 4.3.1. All three substrates showed a characteristic rise-and-fall pattern in cellulase activity, peaking at 72 h and declining thereafter. Wheat bran consistently outperformed the other residues at every time point, reaching 18.3 IU g⁻¹ at 72 h under baseline conditions (pH 5.5, 30 °C, 65% moisture). Rice straw peaked at 14.6 IU g⁻¹, and sugarcane bagasse at 12.1 IU g⁻¹.

Results

Comprehensive Interpretation

The superiority of wheat bran reflects its balanced composition of cellulose, hemicellulose, starch, and protein, which provides both inducer and readily metabolisable nutrients for A. niger. Optimum pH (5.5) and temperature (30 °C) fall within the range typical for fungal cellulases. Nitrogen supplementation with 1% yeast extract boosted activity by 18.6% over the unsupplemented control, though higher concentrations (2%) led to excessive mycelial growth at the expense of enzyme secretion.

Discussion

Wheat bran's superiority as an SSF substrate for cellulase production is consistent with reports by Deswal et al. [9] , who recorded 15.2 IU g⁻¹ with Fomitopsis sp. on wheat bran, and by Bansal et al. [4] , who achieved 16.8 IU g⁻¹ with A. niger on the same substrate. The somewhat higher yield in the present research (18.3 IU g⁻¹) may relate to the higher ambient humidity of the southwestern Nigeria region, which helps maintain substrate moisture during extended incubation [10] . The decline in activity after 72 h coincides with depletion of readily accessible cellulose and accumulation of glucose, a known repressor of cellulase gene expression via carbon catabolite repression [11] . Rice straw, despite its abundance, yielded lower cellulase than wheat bran. This is likely due to the higher silica content of rice straw (12-15% SiO₂), which encrusts cellulose fibrils and physically impedes fungal penetration [3] . Pre-treatment with dilute alkali could potentially remove silica and improve enzyme yields, though it would add processing cost [12] . Sugarcane bagasse, with its higher lignin fraction (22-25%), presented a similar access barrier [13] . From an application standpoint, a cellulase titre of 18-22 IU g⁻¹ is adequate for on-farm saccharification of crop residues into fermentable sugars, provided enzyme loading is scaled appropriately [14] . Coupling SSF cellulase production with small-scale bioethanol fermentation could create a closedloop system for residue valorisation in rural settings.

Conclusion

The present research set out to compare three agricultural residues as substrates and to optimise SSF conditions for cellulase production by A. niger MTCC 2196. Wheat bran emerged as the best substrate, yielding 18.3 IU g⁻¹ ds at 72 h under optimum conditions of pH 5.5, 30 °C, and 65% moisture. Addition of 1% yeast extract raised the yield to 21.7 IU g⁻¹. Rice straw and sugarcane bagasse, though less productive, still generated usable cellulase levels and represent low-cost alternatives where wheat bran is scarce. From a practical standpoint, the optimised SSF protocol is simple enough for adoption in small-scale bioprocessing units. Future work should explore combinatorial substrates, response surface methodology for multi-factor optimisation, and downstream concentration of the crude enzyme for pilot-scale saccharification trials.

References

1. Klein-Marcuschamer D, Oleskowicz-Popiel P, Simmons BA. The challenge of enzyme cost in the production of lignocellulosic biofuels. Biotechnology and Bioengineering. 2012;109(4):1083-1087.
2. Pandey A, Soccol CR, Mitchell D. New developments in solid state fermentation: bioprocesses and products. Process Biochemistry. 2000;35(10):1153-1169.
3. Binod P, Sindhu R, Singhania RR. Bioethanol production from rice straw: an overview. Bioresource Technology. 2010;101(13):4767-4774.
4. Bansal N, Tewari R, Soni R, Soni SK. Production of cellulases from Aspergillus niger NS-2 in solid-state fermentation on agricultural residues. Waste Management. 2012;32(7):1341-1346.
5. Singhania RR, Sukumaran RK, Patel AK, Larroche C, Pandey A. Advancement and comparative profiles of cellulase production. Bioresource Technology. 2010;101(12):4853-4859.
6. Sukumaran RK, Singhania RR, Mathew GM, Pandey A. Cellulase production using biomass feed stock and its application in lignocellulose saccharification. Renewable Energy. 2009;34(2):421-424.
7. Mandels M, Weber J. The production of cellulases. Advances in Chemistry Series. 1969;95:391-414.
8. Ghose TK. Measurement of cellulase activities. Pure and Applied Chemistry. 1987;59(2):257-268.
9. Deswal D, Khasa YP, Kuhad RC. Optimization of cellulase production by a brown rot fungus Fomitopsis sp. RCK2010. Bioresource Technology. 2011;102(10):6065-6072.
10. Raimbault M. General and microbiological aspects of solid substrate fermentation. Electronic Journal of International Journal of Agriculture and Food Science https://www.agriculturaljournals.com Biotechnology. 1998;1(3):174-188.
11. Amore A, Giacobbe S, Faraco V. Regulation of cellulase and hemicellulase gene expression in fungi. Current Genomics. 2013;14(4):230-249.
12. Kim S, Dale BE. Global potential bioethanol production from wasted crops and crop residues. Biomass and Bioenergy. 2004;26(4):361-375.
13. Sun Y, Cheng J. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresource Technology. 2002;83(1):1-11.
14. Saha BC. Hemicellulose bioconversion. Journal of Industrial Microbiology and Biotechnology. 2003;30(5):279-291.